An audio output circuit is used for amplifying a weak audio signal to drive an electro-acoustic transducer such as a speaker or a headphone. FIG. 1 is a conventional circuit diagram showing a configuration of an audio output circuit 100r including a D-class amplifier D. The audio output circuit 100r includes a pulse modulator 110, a first driver 112, a second driver 114, a first D-class amplifier 116, and a second D-class amplifier 118. The pulse modulator 110 pulse-width modulates or pulse-density modulates an audio signal S1. Pulse-modulated audio signals (hereinafter, referred to as pulse signals) S2p and S2n are input to the first driver 112 and the second driver 114, respectively.
An electro-acoustic transducer 2, which is a load, is BTL (Bridge Transless) connected to the first D-class amplifier 116 and the second D-class amplifier 118. A first filter 104 is inserted between a positive terminal (+) of the electro-acoustic transducer 2 and an output of the first D-class amplifier 116), and a second filter 106 is inserted between a negative terminal (−) of the electro-acoustic transducer 2 and an output of the second D-class amplifier 118. Each of the filters 104 and 106 is a primary filter including a series inductor L1 (L2) and a shunt capacitor C1 (C2).
The first driver 112 switches a high-side transistor and a low-side transistor of the first D-class amplifier 116 complementarily in response to the pulse signal S2P. Similarly, the second driver 114 switches a high-side transistor and a low-side transistor of the second D-class amplifier 118 complementarily in response to the pulse signal S2n. 
FIG. 2 shows waveform diagrams in a differential operation of the audio output circuit 100r shown in FIG. 1. Horizontal axes and vertical axes of waveform diagrams and timing charts in the present disclosure may be enlarged or reduced, and each waveform may also be simplified as appropriate for ease of understanding.
A case where the pulse signals S2p and S2n are generated by comparing a triangular wave and the audio signal S1 is described with reference to FIG. 2. The pulse signals S2p and S2n are reverse phased in the D-class amplifiers 116 and 118. Therefore, an amplitude of a differential signal Vo (=Vo+−Vo−) determined based on output voltages Vo+ and Vo− is twice a power supply voltage VDD of the first D-class amplifier 116 and the second D-class amplifier 118
The first filter 104 and the second filter 106 function as low pass filters to reproduce the original audio signal S1 by removing a switching frequency of the differential signal Vo.
Recently, a filterless operation has been adopted instead of the differential operation of the audio output circuit 100r described in FIG. 2. FIG. 3 shows waveform diagrams when the filterless operation of the audio output circuit 100r of FIG. 1 is performed. The pulse signal S2p is generated by comparing the audio signal S1 and the triangular wave and the pulse signal S2n is generated by comparing an inversion signal #S1 of the audio signal S1 and the triangular in the filterless operation. The amplitude of the differential signal Vo applied to the electro-acoustic transducer 2 is half the amplitude of the case where the differential operation is performed. This method is referred to as the filterless operation or a filterless method, since it needs no low pass filter for removing the switching frequency of the differential signal Vo. However, for the purpose of suppressing a spurious radiation (EMI: Electromagnetic Interference), the first filter 104 and the second filter 106 are adopted to function as EMI-removing filters.